The present invention is directed to a hammer drill with a striking mechanism including a free piston slidably displaceable in a reciprocating manner within a guide cylinder for transmitting blows to a tool mounted in one end of the cylinder. Balls mounted in the guide cylinder are spring biased radially inwardly into a recess in the free piston for retaining the piston in a locked position incapable of delivering blows to the tool.
In known hammer drill, striking mechanisms are used to direct blows or percussive force against a tool so that the tool can carry out its intended purpose. So-called pneumatic striking mechanisms have been found to be particularly effective and such mechanisms employ a free piston which can be provided with reciprocating motion by a drive piston by varying the pressure of the drive piston acting on the free piston. As the drive piston reciprocates, it moves the free piston in a similar manner so that the free piston moves back and forth and transmits blows or percussive force to a tool mounted in the hammer drill. It is important in such hammer drills to prevent the transmission of blows to the tool when it is not being used by securing the free piston in a locked position while the drive piston continues to reciprocate, particularly to conserve the tool when not in use.
To secure the free piston in the locked position, it is displaceable toward the tool beyond the working stroke where blows are applied to the tool in a hammer drill, such as disclosed in DE-PS No. 2 806 611. When the free piston experiences this additional displacement movement, a circular recess in the piston moves into the effective range of balls displaced radially inwardly under spring action. The balls snap into a recess in the free piston and hold the piston in a locked position where it cannot transmit percussive force to the tool. To release the free piston from its locked position, it can be directly or indirectly displaced by the tool into a working position. For the inward movement of the balls, an elastic O-ring presses the balls radially inwardly. The balls and the O-ring are mounted in a guide annulus adjacent to the guide cylinder. Such an arrangement is disadvantageous especially due to the high percussive-like force applied to the O-ring when the free piston moves into the effective range of the balls which are displaced with a high velocity. This action leads to the fast wear of the O-ring. A further disadvantage of the known arrangement is that it precludes a radially compact construction, since the balls and the O-ring are located in a guide annulus and provision must be made for radial spring travel. Moreover, an O-ring has a highly progressive spring characteristic as well as a great scattering effect on the spring force.